Corrosion is a natural process that involves a metal atom M being oxidized, whereby it loses one or more electrons and leaves the bulk metal, M→Mm++m e−. The lost electrons are conducted through the bulk metal to another site where they reduce (i.e. combine with) a reducible species such as a dissolved gas or a positively charged ion G+ that is in contact with the bulk metal, N+n e−→Nn− and Gm++m e−→G.
In corrosion parlance, the site where metal atoms lose electrons is called the anode, and the site where electrons are transferred to the reducible species is called the cathode. These sites can be located close to each other on the metal's surface, or far apart depending on the circumstances. When the anodic and cathodic sites are continuous, the corrosion is more or less uniform across the surface. When these sites are far apart, the anodic sites corrode locally.
A corrosion path is essentially an electric circuit, since there is a flow of current between the cathode and anode sites. In order for a current to flow, Kirchoff's circuit laws require that a circuit be closed and that there exists a driving potential (or voltage). Part of the corrosion circuit is the base metal itself; the rest of the circuit exists in an external conductive solution (i.e. an electrolyte) that must be in contact with the metal. This electrolyte serves to take away the oxidized metal ions from the anode and provide reduction species (either nonmetalic atoms or metallic ions) to the cathode. Both the cathode and anode sites are immersed in an electrolyte for the corrosion circuit to be complete.
In corroding systems, potential gradients can be created by a number of mechanisms. These include differences in the free energy or the related electrochemical potentials for different reactions and gradients in the concentration of charged species in the solution. When two electrodes exhibiting differing potentials are electrically connected, a current flows in the external circuit.
There are various approaches to monitoring corrosion; electrochemical approaches rely on the above-described electrochemical corrosion principles and the measurement of potentials or currents to monitor corrosion damage.
One approach to monitoring corrosion is an electrical noise method, which uses electrodes to detect electrochemical noise due to localized corrosion. This method has been implemented using a single pair of near identical large electrodes, and measuring the current noise between the two electrodes. With two large electrodes, each may have a number of anodic areas and a number of cathodic areas, resulting the possibility of zero current flows between the two electrodes. In general, the overall current noise method is not well suited to indicating corrosion rate at a particular site of the metal.
U.S. Pat. No. 6,132,593 to Tan, entitled “Method and Apparatus for Measuring Localized Corrosion and Other Heterogeneous Electrochemical Processes”, describes a multi-sensor electrode, comprising a number of wire beams. This multi-sensor electrode simulates a conventional one-piece electrode surface. Measurements are made by inserting a zero-resistance ammeter between a terminal of a selected wire and the coupled terminals of all other wires. Multiple measurements provide a current distribution map of electrochemical responses on the contact surface of the electrode.
When it is not practical to directly test the component of interest itself, separate sensors can be installed in the same environment. These sensors test a sample of the same material as the component of interest and can be removed from the main component structure and examined in detail. The use of such sensors facilitates the measurement of corrosion damage in a well-controlled manner over a finite sensor area.